1. Field of the Invention
The present invention relates to a method of producing a semiconductor device and, more particularly, to a method of producing a semiconductor device in which fine metallic wires, except for gold wire, are used for the purpose of wiring between electrodes of a semiconductor chip and external leads, with a higher reliability than that of semiconductor devices using gold wire bonding.
2. Description of the related Art
FIGS. 1 and 2 are sectional views of semiconductor devices produced through known processes, particularly electrode structures of the device. Each semiconductor device 1, 1a has an Si substrate 11 on which is formed an SiO.sub.2 film 12 which in turn is overlaid by an Al film 13.
The Si substrate 11 is bonded to a die pad 14 of a lead frame by a bonding material such as an epoxy resin 16 or an Au-Si solder 17. A fine metallic wire 21, which is in this case a copper wire, was melted at its end in order to form a copper ball 22. The copper ball 22 is pressed against the AL film 13 by means of a capillary Tip (not shown) in order to deform the ball plastically. Meanwhile, ultrasonic energy is applied together with heat energy, to a temperature of 250.degree. to 400.degree. C., from the Si substrate 11, so that an intermetallic compound is formed between the Al film 13 and the copper ball 22. More specifically, the intermetallic compound phases 31, 32 and 33 are, respectively, the .theta. phase (CuAl.sub.2), .eta..sub.2 phase (CuAl.sub.2) and .gamma..sub.2 phase (Cu.sub.9 Al.sub.4) of the copper and aluminum. This semiconductor device is then sealed with ceramic having an empty room for the chip or molded with resin. It seems that the .theta. phase, layer 31 shown in FIG. 1 is a non-uniform alloy layer the thickness of which fluctuates largely, whereas the .theta. phase 31, .eta..sub.2 phase 32 and .gamma..sub.2 phase 33 shown in FIG. 2 are uniform alloy layers with little fluctuation in thickness.
The intermetallic compound of the alloy layer can be identified by the alloy layer phase color: namely, by etching the Al layer 13 with phosphoric acid (H.sub.3 PO.sub.4), causing the copper ball 22 to color by treating the same with an aqueous solution of sodium hydroxide (NaOH), and identifying the .theta. phase layer 31 by a brown color at the bonding face.
FIG. 3 is a schematic illustration of bonding face of the copper ball 22 shown in FIG. 1. In this case, the brown color (.theta. phase) 31 does not exist uniformly over the entire area and non brown colored portions where the .theta. phase is not formed are observed here and there.
FIG. 4 is a schematic illustration of bonding face of the copper ball 22 shown in FIG. 2. The brown color (.theta. phase) is spread over the entire area, and the white and blue color .eta..sub.2 phase 32 or .gamma..sub.2 phase 33 exist in local areas.
Japanese Published Patent Application 62-265729 discloses a semiconductor device. Judging from the description in the specification, it is understood that the structure of this device is similar to those shown in FIGS. 2 and 4, i.e., the .theta. phase (CuAl.sub.2), .eta..sub.2 phase (CuAl) and .gamma..sub.2 phase (Cu.sub.9 Al.sub.4) exist in the alloy layer.
The known semiconductor device has an above bonding structure. Then it is sealed with ceramic having an empty room for the chip or molded with resin.
The reliability of the semiconductor device is evaluated by an accelerated reliability test. The empty ceramic seal package device has little problems. However, when the device is molded in epoxy resin and especially during the high temperature (250.degree. C.) storage test, the reliability of this device is subject to larger fluctuations than that of the device in which gold wire is used. In addition, the device life during high temperature storage test is substantially equivalent to the device using gold wire used device, thus the expectation that the copper wire can expand the operational margin in higher temperature region over the gold wire.